Research in animal cognition has provided evidence that species as diverse as monkeys and pigeons are capable of the formation of stimulus classes that are necessary for the acquisition of concepts including those required for the development of human language (Zentall & Smeets, 1996). Although there is now good evidence for the evidence of the existence of such coding processes little is known about the nature of the resulting codes. A significant limitation of animal models of cognitive learning has been the difficulty in specifying how animals represent events in memory. The long-term objects of this project are to determine the mechanisms responsible for these coding processes and to identify the nature of the codes themselves. At a practical level, these procedures can be (and already have been) used in the treatment of learning disabled and developmentally delayed children (Sidman, 1994). Recent evidence suggests that when pigeons acquire a delayed conditional discrimination involving the presence versus the absence of a conditional stimulus, they do so by developing a single-code/default strategy (one of the two conditional stimuli is coded, the other is not, but is assumed to have occurred in the absence of a code for the first). Evidence for the development of such a strategy comes in the form of non-parallel (divergent) retention function obtained when delay is inserted between the initial (conditional) stimuli and the test stimuli. The high, flat retention function on absent-sample trials and step retention function (falling below chance) on present-sample suggests that only the present sample is represented in memory and a response indicating that a absent sample has been present is made by default. An understanding of the development of the single-code/default strategy is important because it suggests that pigeons have the capacity to acquire an inherently symmetrical (i.e., two-alternative) task using an asymmetric, but perhaps more efficient, coding strategy. The purpose of the present research is (1) to critically evaluate existing evidence for the development/the single-code/default strategy in pigeons and to assess alternative hypotheses concerning the determinants of the divergent retention functions, (2) to determine the conditions under which true single- code/default strategies will develop, and (3) to determine the basis for retention functions obtained following duration-sample matching (research on memory for duration), which are strikingly similar to those following training with present-absent samples nature of that single underlying code.